Prior to the present invention, as shown by Wright et al., U.S. Pat. No. 3,664,899, assigned to the same assignee as the present invention and incorporated herein by reference, a method for removing organic polymeric film from a substrate was provided utilizing ultraviolet light at wavelengths below 3500 Angstroms in the presence of oxygen. Although the dry film removal process of Wright et al. represented a significant advantage over wet film removal methods utilizing organic solvents and mechanical action such as scraping, the patterned removal of photoresists from various substrate by the method of Wright et al. nevertheless required several minutes.
An improved procedure for providing patterned removal of organic material from applied organic photoresist, useful in the fabrication of microelectronic devices, is shown by Blum et al., U.S. Pat. No. 4,414,059. Blum et al. utilize ablative photodecomposition (APD) and radiation at a wavelength of 193 nanometers. In order to achieve sufficient power density (or energy fluence), a pulsed radiation source is used, such as produced by an ArF excimer laser providing pulses approximately 10 nanoseconds wide at 193 nanometers. The critical threshold energy fluence is approximately 10-15 mJ/cm.sup.2 per 10 nanosecond pulse. According to Blum et al., ablative photodecomposition leads to extremely high etch rates, being at least 30 times as great as those obtainable in oxidative removal processes.
Even though significant advances in rate of patterned removal of organic material from organic films has been achieved, improvements in the removal rate of organic material in a patterned manner from organic film on a substrate are constantly being sought.
The present invention is based on our discovery that a substantial enhancement in the rate of patterned organic material removal from applied organic films can be achieved by modifying the composition of the applied organic films. More particularly, we have found that a substantial enhancement in the rate of removal of organic material in a patterned manner from an applied organic film can be achieved by utilizing organic polymeric film made from a blend of aliphatic organic polymer and aromatic organic material, or aromatic organic copolymer consisting essentially of chemically combined aliphatic organic units and aromatic organic units. In order to achieve optimum removal rate of organic material, we have found that aromatic organic units must be present in the blend or copolymer at particular proportions by weight, as defined below.
As defined hereinafter, the expression "removal rate of patterned organic material" means the depth in micrometers ablatively etched, per incident pulse. The pulse duration for the ArF laser at 193 nanometers is 12 nanoseconds. The removal rate can be determined by dividing the total thickness of etched material by the total number of incident pulses. As shown in the drawing, an excimer laser operated at a wavelength of 193 nanometers and a power level of 0.1 to 0.5 joules/cm.sup.2 is capable of effecting a removal rate of between about 0.1 .mu.m to 0.5 .mu.m, per pulse, of various organic films, where "PS" is poly(.alpha.-methylstyrene), PMMA is polymethylmethacrylate and 2% and 20% is in weight percent. Although the laser is capable of a repeat rate of 25 pulses per second, it was operated at 1 pulse per second in the evaluation. Hence, the rates shown are in thicknesses removed per second. These values would be increased if a higher excimer laser pulse repetition rate were used.